When it comes time to find a mate, some fish are perfectly happy to look for love beyond the bounds of their own species. But those that aren’t can be particularly picky, even among their own kind.

A new paper from the University of Illinois shows that female killifish that won’t mate with males of another, related species are more discerning when choosing mates among unfamiliar populations of their own species, according to a university press release. The study is the first of its kind to observe this effect in fish.

Reinforcement is a process of natural selection that favors traits increasing isolation between differentiated, interbreeding populations. When members of two species mate, their hybrid offspring often possess inferior traits and a lower likelihood of survival. Fish that mate with their own species, however, have a better chance of producing offspring that live to adulthood. When reinforcement occurs between populations of a common species, the phenomenon is known as cascade reinforcement.

While rainwater killifish and bluefin killifish prefer habitats of different salinities, they’re adaptable enough to share a habitat at times. The study found that when both species live in close proximity, female rainwater killifish will actively avoid males of their species that belong to another population.

The short-term benefits to survival — and the long-term genetic benefits — of reinforcement are easy enough to observe. One Norwegian study showed that when escaped farm salmon breed with local wild salmon, negative impacts to lifetime success, individual fitness and population production can be observed over at least two generations. But according to UI professor Rebecca Fuller, lead author of the new paper, the benefits of cascade reinforcement are unclear — if there are any at all.

Cascade reinforcement could mark the early stages of the development of new species, Fuller said in a UI press release, but more research is necessary to determine if this is true.

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